DISTRIBUTION AND AGE OF DUCTILE STRAIN BETWEEN TWO DETACHMENT FAULTS IN THE FOOTWALL OF THE PIONEER METAMORPHIC CORE COMPLEX

The Eocene Pioneer core complex (PCC) of south-central Idaho is bound by the Wildhorse detachment; internally, a slightly older detachment divides the footwall into a middle and lower plate. This study uses detailed field mapping and zircon U-Pb geochronology to bracket the age of extension and the distribution of strain in the middle plate in the region where the two detachments converge. A WNW-trending elongation lineation is pervasive in the middle plate. In our area, however, structural contrasts define upper and lower domains. Outcrop and larger-scale folds with WNW-tending fold axes, SW-dipping axial planar foliations with WNW lineations, and planar undeformed leucocratic dikes characterize the upper structural domain. In the lower domain, planar units dip SW, large folds are absent, and dikes are both folded and planar. Strain appears to be greater in the lower domain based on a highly thinned stratigraphy, widespread boudinage, and deformed Eocene dikes. These dikes are isoclinally folded and/or boudinaged indicating emplacement before or during regional extension, and planar cross-cutting dikes mark the end of extension. The overall strain observed here—at the convergence of the two PCC detachments—is higher than sections in similar structural positions of the core complex, perhaps suggesting the high strains result from cumulative strain related to both detachments. The upward decreasing strain toward the Wildhorse detachment suggests a dominate role for the lower detachment. Four pairs of deformed and undeformed dikes were sampled to bracket the age of deformation at different levels in the highly deformed lower domain, but only two samples produced viable igneous ages. An isoclinally folded dike within a banded calc-silicate unit yielded an age of ~47.7 Ma and an undeformed dike intruding multiple units yielded an age of ~43.8 Ma. These ages indicate that extension-related deformation was occurring at or after ~47.7 Ma and concluded by ~43.8 Ma in the middle plate. When combined with the ~>49 Ma to ~47 Ma age range of deformation in the deeper lower plate, our ages indicate that strain localized within the middle plate level after it ended at deeper levels in the exposed footwall.